1. Field of Invention
The invention relates to high electron mobility transistors (HEMTs) and metamorphic HEMTs (MHEMTs).
2. Related Art
HEMTs are active semiconductor devices that are identified by high electron mobility in the channel (e.g., above 6,000 volt/cm2-sec.). HEMTs typically use compounds containing indium for the channel, since the electron mobility for indium arsenide is higher than in many other elements and compounds used to construct transistors (e.g., silicon, gallium arsenide). This high electron mobility allows the device to operate at frequencies higher than other (e.g., silicon based) semiconductor devices. Further, HEMTs provide low noise in very high frequency (e.g., 100 gigaHertz (GHz)) applications. Due to these desirable characteristics, HEMTs are replacing metal semiconductor field effect transistors (MESFETs) in high speed communication devices (e.g., switches).
Transistors have an intrinsic gain roll-off as the frequency of the signal applied to the gate (input signal) increases. A typical gain roll-off is about xe2x88x9220 dB per frequency decade (e.g., 1 GHz to 10 GHz) or xe2x88x926 dB per octave (frequency doubling). A transistor""s transition frequency (FT) is the frequency at which unity current gain occurs for a particular bias. Bias may be optimized for a transistor to provide the highest possible FT. The maximum FT for current HEMTs is about 200 GHz.
Since electron speed in semiconductor material is limited, transistor dimensions (e.g., gate length) are decreased to provide higher switching (e.g., on-off) speeds. But low sub-micron transistors (e.g., minimum feature size below 0.25 micrometer (xcexcm)) often experience unacceptably high off-state current. This high off-state current problem is called the short channel effect, and it results from an inability to effectively pinch off the channel during the off-state. This short channel effect acts to limit transistor switching speeds, and current HEMTs suffer from this short channel effect.
Therefore, what is required is a HEMT with decreased short channel effect (lesser off-state current) to allow the HEMT to operate with an FT above 200 GHz.
An illustrative high electron mobility transistor is constructed with a substrate, a buffer layer formed on the substrate, and a heavily doped p-type barrier layer formed on the buffer layer. In one illustrative embodiment, the substrate is gallium arsenide (GaAs) and the buffer layer is a relaxed lattice metamorphic layer containing indium. In a second illustrative embodiment, the substrate is indium phosphide (InP) and the buffer layer is a lattice matching indium aluminum arsenide layer. A spacer layer is formed on the barrier layer, and a channel layer is formed on the spacer layer. The channel layer may be of uniform composition, or may be made from two or more sublayers. A Schottky layer is formed over the channel layer, and source and drain contacts are formed on the Schottky layer. The transistor is formed to have a channel length of 0.25 xcexcm or less (e.g., 0.15 xcexcm or less).
In the illustrative HEMT embodiments, one or more of the HEMT structure layers overlying the substrate contain indium to provide high electron mobility. A heavily doped p-type barrier layer improves electron confinement within the channel layer, thereby eliminating the short channel effect at frequencies above 100 GHz and allowing enhancement-mode operation by minimizing surface depletion effect. This structure provides an FT above 200 GHz. A lattice-relaxed buffer layer allows the transistor to be formed using a GaAs substrate, with a resulting manufacturing cost decrease due to the larger size, lower cost, and robustness of gallium arsenide wafers. Alternatively, a HEMT is formed over a substrate that contains indium or other semiconductor materials.